KLEEFSTRA R, DE JONG, J & SCHAUB T (2023) Breeding of Short-eared Owls Asio flammeus in fields full of Common Voles Microtus arvalis in the province of Friesland in 2019. LIMOSA 96 (2): 60-71.

When in 2014 an outbreak of Common Voles led to an invasion of about 50 breeding pairs of Short-eared Owls in the Frisian agricultural grasslands, it was believed to be a once-in-a-lifetime natural history event. In the winter of 2018/19, however, signs of a vole outbreak became apparent and already in March the first Shorteared Owls started breeding. A total of 85 probable breeding cases was reported. The breeding period covered over half a year, from the end of February (first eggs) to almost mid-September (fledging of the last young). In 10 cases there was evidence for second clutches (pairs starting a new clutch shortly after a failed breeding attempt in the vicinity of the first nest), bringing the number of territories to at least 75 (Tab. 1). This number is significantly higher than in 2014 and accounts for the highest number of breeding Shorteared Owls in the province Friesland since 1990 (Fig. 1). The Short-eared Owls spread widely over the western and northern half of the province (Fig. 2). As in 2014, slightly more than half of the nests were located on peat soil (56.0%; Tab. 1) and by far the vast majority (90.5%) bred in intensive agricultural grassland with perennial ryegrass.
The average number of eggs in 34 complete clutches was 7.5 eggs per nest (Tab. 2). Compared to 2014, clutches were larger by on average two eggs per nest. Based on nests from which this could be calculated, Short-eared Owls started laying between 21 March and 2 July (except one clutch that must have been started as early as late February). Calculated over all clutches egg laying started 23 days earlier than in 2014. The first young of 13 nests hatched between 14 April and 28 July, with 5 June as the average date. Of 231 eggs in 38 nests, at least 115 (49.8%) hatched in 17 nests. This is lower than in 2014 when 56.3% of 167 eggs hatched. The number of young that fledged was 98-150, of which 94 were ringed. This is more than in 2014 when 82-112 young fledged, 64 of which were ringed. Although the number of territories and the number of fledglings in 2019 was higher than in 2014, the number of successful nests was the same (Tab. 3). The chick condition was similar to that in 2014 (Fig. 3). The reason for nest failures was known for 31 (partly first) breeding attempts (Tab. 3). At least 14 nests were predated, of which five within a day after mowing of the grass. In three cases the predator was a small marten, in one case a Carrion Crow and in one case the breeding female was predated by Red Fox or Beech Marten. Among nine abandoned clutches, three were no longer incubated immediately after mowing. Of eight nests the cause of failure could not be determined with certainty, but mowing is the most likely cause. In at least one case, a brooding female was killed during mowing. The picture on causes for breeding failures does not differ much from 2014 (Tab. 3).
During nest visits, a total of 101 pellets were collected, in which the majority (95.8%) were vole and the other prey also consisted of small mammals (Tab. 4). In one region, 24 fresh, intact voles with an average weight of 26.5 grams (range 15-35 grams), were found during three nest visits, divided over two nests. Short-eared Owls nested at locations with relatively high densities of Common Voles (Tab. 5). This is expressed not only in the average number of holes per square meter (plot), but also in the percentage of plots in which fresh tracks of vole were present. This was the case in about 85% of the 360 plots. In autumn the density of holes had doubled.
We tagged an adult male Short-eared Owl on 18 July, 2019. It stayed at the nesting site until the end of August (its chicks then 8-9 weeks old). He then moved 2 km to the west, only to move two weeks later over 4 km east of the nesting site, where he stayed for at least a month. During the nesting period (18 July to 30 August) he had a small home range of 44 ha with a core area of 3 ha (Fig. 4). 97.5% of positions were within 500 m of the nest; the maximum distance was 2.2 km (median 52 m). The transmitter’s battery ran out in mid-October. When the transmitter reconnected in January 2020, it turned out that the Short-eared Owl had moved to Libya since 11 December, 2019. At the end of January, it moved to the border area between Tunisia and Algeria. In early April 2020, it took off in a north-easterly direction and flew in a straight line via Sicily and southern Italy, through the Balkans to Russia (Fig. 5). Between the end of May and the end of June it was stationary in a small area in Saratov Oblast (about 600 km south-east of Moscow). It is conceivable that he made a (failed) breeding attempt here, but without field observations this is uncertain. The last GPS position was from Bulgaria on 25 November, 2020. After that, the transmitter made regular GSM connections, but without sending GPS coordinates. From the last two connections in August 2021, it could be derived that the owl was again in Russia (Russian network provider). It is not certain whether the Short-eared Owl was still alive at the time.

[pdf only for members] [dutch summary]